The present invention relates to a process for coating a metallic surface with a vitreous enamel which process does not require pre-treatment of the metal surface.
Enamel coated metallic surfaces are commonly used in household appliances because of their durability, resistance to corrosion and pleasing appearance. In the production of enamel coated sheet metal, one of the major objectives is a smooth surface with no blistering or bubbling. In order to achieve this objective, it was believed that the metal surface to be coated must be scrupulously clean and that pre-treatment to promote adhesion of the enamel coating to the metal (i.e. acid etching and flashing with nickel) was necessary.
Frits and processing techniques which eliminate the need for etching and flashing have been developed (See e.g. U.S. Pat. No. 4,221,824). Cleaning, however, was still required because in forming the metal to be coated, the metal surface is generally covered with an oil. Removal of such oil is necessary because oils pyrolyze at temperatures lower than the fusion temperature for the enamel-forming material and leave a carbonaceous residue on the metal surface. This residue detrimentally affects the surface characteristics of the coated metal by causing blistering and bubbling. This cleaning has generally been accomplished by chemical methods which require alkali and soap solutions for removing grease and dirt and acid solutions for removing oxidized metal. Such cleaning requires close control and the use of chemicals or energy. Disposal problems as well as the labor and equipment required, necessarily increase production time and cost. It would therefore be advantageous to be able to eliminate the need for such cleaning pre-treatment in a manner which would not result in surface defects.
One approach to eliminating the need for cleaning the metal surface to be coated without sacrificing surface characteristics is disclosed in U.S. Pat. No. 3,906,125. In this patent, a process for treating sheet steel in which the steel is subjected to oxidation, reduction and decarburization before the enamel-forming material is applied is described. Cleaning of the steel to remove oil is unnecessary in this process if the enamel-forming material is applied immediately after the decarburization. Where such immediate use is not possible, the treated steel is given a temporary coating of oil which oil must be subsequently removed. Consequently, the cleaning step is avoided in this process only if the steel is subjected to more extensive pre-treatment and the enamelling operation is carried out immediately after milling. Since immediate use is generally not possible, this disclosed process does not provide a practical alternative to coating the metal surface with an oil.